A pulse width modulation (PWM) power amplifier is generally utilized, e.g., as a power supply or an amplifier. In this PWM power amplifier, a PWM switching circuit is employed as a power converter circuit, and an LC filter for eliminating noises is inserted between the power converter circuit and a load, and further a feedback control system is configured so that an output voltage supplied to the load becomes proportional to a command signal. At this time, the characteristics of the load to which the PWM power amplifier is applied range as widely as from the capacitive to the inductive, while the magnitude of the load varies to a large extent from a zero to a maximum of an amplifier's rating. Consequently, a robust PWM power amplifier is needed which can respond to such wide-range variations in load and in DC power supply voltage using one controller.
Designing methods for an analogue controller in such a robust PWM power amplifier are disclosed, e.g., in “The robust design of PWM power amplifier by the approximate 2-degree-of-freedom integral type servo system”, Proc. IEEE IECON-2000, pp. 2297-2302, 2000; and K. Higuchi, K. Nakano, K. Araki, and F. Chino, “The robust design of PWM power amplifier by the approximate 2-degree-of-freedom digital integral type control system”, The journal of the Institute of Electrical Engineers of Japan, Vol. 122, No. 2, pp. 96-103, 2002. In the methods, however, current feedback and voltage feedback, on which carrier noises are superimposed, are utilized in controllers. In order to suppress noise effect on the controller, however, an amount of feedback signals should desirably be small and a current detecting sensor is generally expensive, and thus a controller using only the voltage feedback is desirable. In that case, since no current feedback is employed, a configuration of the analogue controller becomes of complexity which leads difficulty to realize the configuration. In the case of the digital controller, however, the use of DSP (Digital Signal Processor) can facilitate realization of the digital controller.
Then, the designing method of the robust digital controller in the PWM power amplifier that satisfies the foregoing demands is proposed in K. Higuchi, K. Nakano, K. Araki, and F. Chino, “The robust design of PWM power amplifier by the approximate 2-degree-of-freedom integral type control system only with voltage feedback”, The journal of the Institute of Electronic, Information and Communication Engineers, Vol. J-85-C, No. 10, pp. 1-11 (2002.10) (“Higuchi et al.”).
In the digital feedback control system, there occurs an input dead time longer than in the analogue feedback control system. This input dead time is attributable mainly to a calculation time-lag of DSP, a conversion time from an analog signal to a digital code (AD conversion) and a conversion time from a digital code to an analog signal (DA conversion), a time-lag at a triangle wave comparison section or the like. Focusing on this point, by taking into consideration the input dead time and the conversion of the current feedback into the voltage feedback, the foregoing Higuchi et al. expressed control targets (a PWM signal generating section, a power converter circuit and an LC filter) with a discrete-time system whose order is higher by two orders than a continuous-time system, and proposed a state feedback system for attaining target characteristics given for the control target. Further, the foregoing Higuchi et al. proposed that after the state feedback system is converted equivalently into an output feedback system using only voltage, an approximate two-degree-of-freedom digital robust control system can be constituted, when a robust compensator obtained by approximating the output feedback system is connected, while by applying an equivalent conversion to this digital robust control system, a digital integral type controller using only the voltage feedback can be obtained.
In the foregoing documents, a method for constituting an approximate two-degree-of-freedom digital robust control system, which realizes a first-order approximate model, was shown. In a robust digital controller with such a control system incorporated therein, it is difficult to limit the magnitude of a control input while increasing a degree of approximation. Consequently, it was needed to provide a robust digital controller in which a high degree of approximation is easily possible and at the same time there is no need to take the magnitude of the control input into consideration. Further, with regard to the two-degree-of-freedom robust digital controller proposed in the above documents, a clear parameter determining means for enhancing the degree of the approximation of the robust digital controller is not disclosed. Thus, the substantial amount of trial and error was needed to determine the parameters, requiring a large amount of labor hours. Thus, it was needed to provide a designing device having a clear parameter determining means for allowing anyone to design the parameters easily.
The present invention addresses the problems mentioned above. Therefore, the first objective of the present invention is to provide a robust digital controller incorporating a novel two-degree-of freedom robust digital control system with a high degree of approximation and without the need for considering the magnitude of the control input. Further, the second objective of the present invention is to provide a designing device for such a robust digital controller.